The use of nitrogen has become increasingly important in various industrial and commercial operations. For example, liquid nitrogen is used to freeze food, in the cryogenic recycling of tires and as a source of gaseous nitrogen for inerting. Gaseous nitrogen is used in applications such as secondary oil and gas recoveries and as a blanketing gas in metal refineries, metal working operations, semiconductor manufacture and chemical processes. In light of the increasing importance of nitrogen in such operations, it is desirable to provide a process which is both economical and efficient for producing nitrogen.
High purity gaseous nitrogen is produced directly by well known cryogenic separation methods. U.S. Pat. No 4,222,756 teaches a process and apparatus for producing gaseous nitrogen using multiple distillation columns and associated heat exchangers. Ruhemann and Limb, I. Chem. E. Symposium Series No. 79, page 320 (1983) advocate a preference for the use of the single distillation column instead of the typical double column for the production of gaseous nitrogen.
Liquid nitrogen is typically produced by initially producing gaseous nitrogen in a cryogenic air separation unit and subsequently treating the gaseous nitrogen in a liquefier. Modified forms of cryogenic air separation units have been developed to directly produce liquid nitrogen. U.S. Pat. No. 4,152,130 discloses a method of producing liquid oxygen and/or liquid nitrogen. This method comprises providing a substantially dry and substantially carbon dioxide-free air stream, cryogenically treating the air stream to liquefy a portion of the air stream, and subsequently feeding the air stream into a fractionation column to separate the nitrogen and oxygen and withdrawing liquid oxygen and/or nitrogen from said column.
Various process cycles using a single distillation column, with some boil-up at the bottom provided by the appropriate high pressure fluids, have also been suggested in the patent literature, for example, U.S. Pat. No. 4,400,188 and U.S. Pat. No. 4,464,188.
In U.S. Pat. No. 4,595,405 a process for the cryogenic separation of nitrogen from air is taught, wherein the cryogenic separation is conducted in a single pressure distillation column. The oxygen enriched waste gas from the cryogenic separation is rewarmed, compressed to an elevated pressure and processed through a selective membrane separation to extract oxygen from the waste stream for recovery or removal, while returning a nitrogen enriched stream to the cryogenic separation. This process entails the additional capital outlay for membrane separation. It would be logical in that patented process, designed for the recovery of nitrogen, to recycle a nitrogen-enriched stream, after membrane treatment to remove its predominantly oxygen content, as is performed in that patent.
In many of the cryogenic processes for recovery of nitrogen, the oxygen-enriched waste stream is removed from the cryogenic separation zone or distillation column and is reduced in pressure with the recovery of work in order to produce refrigeration for the feed stream being cooled for cryogenic separation. There is a minimum pressure at which such a process may be operated in order to provide sufficient refrigeration for the continuous operation of the process to produce gas. This pressure may be in excess of the pressure at which the product is required and thus there is an energy inefficiency in the production process. A1ternatively, it is sometimes desirable to produce nitrogen at high pressure directly from the distillation system without further compression; for example in the production of nitrogen for the electronics industry. ln this case, the combined flow and pressure of oxygen-enriched waste is often greater than is necessary to reduce in pressure with the recovery of work for the production of refrigeration. In this event, all of such waste cannot be processed accordingly without creating excess refrigeration. To avoid production of excess refrigeration, a portion of the waste stream is merely passed through an expansion valve, without the recovery of work, so as to minimize refrigeration production. This expansion without the recovery of work is a waste of the energy utilized to create the pressurized condition of that stream, as well as a waste of the nitrogen content of the stream.
The present invention overcomes the drawbacks of the prior art in producing high purity nitrogen using a cryogenic separation technique, wherein efficiencies are derived by the use of recycle and pressure maintenance of certain process streams as set forth below.